Jörg Imbrock

Second harmonic generation in multi-domain χ 2 media: from disorder to order

We investigate systematically the evolution of second harmonic generation in strontium barium niobate with different degrees of disorder of its χ2 nonlinearity. These different degrees of disordered domain structures are achieved through electrically switching of the polarization at room temperature. The size and distribution of the domains change during the poling process and this in turn strongly affects the spatial distribution of the second harmonic signal. The degree of disorder can be determined by analyzing the angular distribution and wavelength dependence of the second harmonic emission patterns combined with measurements of the spontaneous polarization. We demonstrate evidence of the control of the second harmonic emission pattern by creating defined states of order, and successfully reproduce the resulting patterns theoretically.

Optically induced photonic superlattices by holographic multiplexing

We present an efficient method for optical induction of photonic superlattices in photorefractive media via holographic multiplexing. By superimposing phase engineered periodic waves of different periodicities, incremental recording of one- and two-dimensional multiperiodic lattices is demonstrated. The induced structures are subsequently analysed in Fourier space as well as in real space to verify the existence of multiple band gaps in the linear transmission spectrum.

Nonvolatile holographic storage in iron-doped lithium tantalate with continuous-wave laser light

Holograms have been recorded in congruent LiTaO(3):Fe with continuous-wave laser light by use of a two-step process. Blue gating light (lambda=488 nm) sensitizes the crystals for holographic recording with red light (lambda=660 nm) of a diode laser. Refractive-index changes of as much as 1.0x10(-5) are achieved for intensities of the red light of 1 W/cm(2) . The saturation values are proportional to the intensity of the writing light. Nondestructive readout with red light is possible, and the holograms remain erasable for blue light.

Refractive Indices of Photorefractive Bismuth Titanate, Barium‐Calcium Titanate, Bismuth Germanium Oxide, and Lead Germanate

The refractive indices of photorefractive bismuth titanate, barium-calcium titanate, bismuth germanium oxide, and lead germanate are measured in the range from 450 to 700 nm using the method of minimum deviation. The results are fitted to the Sellmeier equation and the parameters are given.

Electro–optical tunable waveguide embedded multiscan Bragg gratings in lithium niobate by direct femtosecond laser writing

optical tunable Bragg gratings in lithium niobate fabricated by direct femtosecond laser writing. The hybrid design that consists of a circular type-II waveguide and a multiscan type-I Bragg grating exhibits low loss ordinary and extraordinary polarized guiding as well as narrowband reflections in the c-band of optical communications. High bandwidth tunability of more than a peak width and nearly preserved electro-optic coefficients of r(13) = 7.59 pm V(-1) and r(33) = 23.21 pm V(-1) are demonstrated.

Electro-optical tunable waveguide Bragg gratings in lithium niobate induced by femtosecond laser writing

We report the fabrication of femtosecond laser-induced, first-order waveguide Bragg gratings in lithium niobate in the low repetition rate regime. Type-II waveguides are written into an x-cut lithium niobate wafer and structured periodically to achieve narrowband reflections at wavelengths around 1550 nm. Additionally, electrodes are employed to allow for electro-optic tuning of the spectral response. We demonstrate wavelength control of the central reflection peak by applying a static external electric field. A maximum shift of the reflection peak of Δλ = 625 pm is observed.

Domain-shape-based modulation of Čerenkov second-harmonic generation in multidomain strontium barium niobate

We study experimentally and numerically the second-harmonic Čerenkov emission with two different characteristic azimuthal intensity distributions in strontium barium niobate with a random structure of χ2 nonlinearity. We monitor in situ the Čerenkov emission during domain switching and show that a change of domain size and shape results in a fourfold azimuthal modulation of the Čerenkov cone.

Determination of the composition of lithium tantalate crystals by zero-birefringence measurements

We present an approach to determine the exact composition of lithium tantalate (LiTaO3) crystals by measuring the temperature where the birefringence disappears. Various samples with compositions ranging from the congruently melting to the stoichiometric one have been prepared by vapor transport equilibration treatments and are investigated. The sensitivity is as high as 1.5 °C/0.01 mol % Li2O. The relative accuracy is better than 0.01 mol %, while the absolute accuracy is limited by the calibration.

Photorefractive properties of undoped lithium tantalate crystals for various composition

Lithium tantalate crystals of compositions ranging from 48.3mol%to50.0mol% lithium oxide are fabricated by vapor transport equilibration. Light-induced refractive index changes of the crystals are investigated with holographic methods at usual cw-laser intensities (≈105W∕m2) and with a single focused laser beam at high light intensities up to 2×107W∕m2. In stoichiometric crystals the index changes are reduced by more than two orders of magnitude when compared with congruently melting ones. Simultaneously, the normalized photoconductivity σph∕I, where I is the light intensity, increases by nearly two orders of magnitude. Therefore, stoichiometric lithium tantalate is an attractive material for applications such as frequency conversion via quasi-phase matching.

Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity

We demonstrate second harmonic generation in quasi phase-matched waveguide structures fabricated by direct laser writing. Circular waveguides are inscribed in z-cut lithium niobate that provide well confined guiding of the fundamental and second harmonic wave. In contrast to classic schemes that employ periodically poled crystals, quasi phase-matching is realized by a laser-induced modulation of the nonlinearity inside the waveguide core. The proposed design allows monolithic integration of buried frequency conversion devices with tailored nonlinear response and excellent compatibility to on-chip optical elements. Second harmonic generation of 1064 nm radiation is demonstrated for different grating periods and associated matching temperatures. A maximum conversion efficiency of 5.72% is obtained for a 6 mm long, laser-induced quasi phase-matching grating.